128 Motions of the Stars To further appreciate this apparent change in the situation of the constellations, the student should refer to the large plates successively. In each successive one he will note the advancement westward of the constellations mentioned above, rising in the east late at night. The student can best get an idea of this westward apparent movement of the stars by noting the position of some bright first-magnitude star from night to night. He will soon be able to calculate the position of this star a month or more ahead, and this calculation applies to all the constellations and stars. It is not within the scope of this work to go into this matter in detail. The author merely desires to mention this fact of apparent change of position in the stars, a fact that will be noticeable to the observer in a short time, and a fact that it is hoped he will be able to explain to his own satisfaction with the aid of the foregoing remarks. It will be noticed that the stars on the diagrams are all numbered and lettered. The numbers refer to the magnitude of the star,-that is, the brightness of it, the first-magnitude stars being the brightest, the secondmagnitude stars less bright, etc. The letters are those of the Greek alphabet, and the student if not familiar with it is advised to consult a Greek grammar. In the text, in referring to certain stars in the con Motions of the Stars 129 stellations, the genitive case of the Latin name of the constellation is given; for example, Vega is known as (a) Lyrae, meaning alpha of Lyra, Aldebaran as (a) Tauri, alpha of Taurus, etc. The twilight hour affords an excellent opportunity of fixing the relative positions of the first-magnitude stars in the mind, for at that time they alone, save the planets, are visible. 9 METEORS, OR SHOOTING-STARS. S this work is designed primarily to cover what is observable in the starlit heavens with the naked eye, the subject of meteors, or shooting-stars, comes properly within its scope. There are few persons, if any, who have not witnessed the sight of a splendid meteor speeding across the sky, and such a sight always calls forth exclamations of wonder and delight. Apparently these evanescent wanderers in space are without distinctive features, and baffle classification; but, like all that nature reveals to us, they have been found, for the most part, to conform to certain laws, and to bear certain marks of resemblance that permit of their identification and classification. By careful observation for over fifty years the meteors, generally speaking, have been so arranged that they come under the head of one of the nearly three hundred distinct showers which are now recognized by astronomers. Many of these showers are too feeble and faint to be worthy of the attention of one not especially interested in the subject, but certain ones are well worth observ Meteors, or Shooting Stars 131 ing. There is always a pleasure in being able to recognize at a glance a certain definite manifestation of nature, be it a rare flower or a flashing meteor. The generally accepted theory respecting the meteors is that they were all originally parts of comets now disintegrated, and the four well-known showers of April 20th, August 10th and 14th, and November 27th, bear testimony to this theory. The apparent velocity of the meteors is between ten and forty-five miles a second, and their average height is about seventy-six miles at first appearance, and fifty-one miles at disappearance. Occasionally a meteor is so large and compact as to escape total destruction, and falls to the earth. Specimens of these meteorites are to be found in our best museums. I have seen fit to divide the principal meteor showers into four groups, according to the seasons in which they appear, and have placed them respectively at the conclusion of each season's work on the constellations. By radiant point is meant the point from which the meteors start on their flight. This point is an apparent one, however, due to an illusion of perspective, for the meteors really approach the earth in parallel paths. The dates given for these showers are those of the maxima, and the meteors should be looked for several nights before and after the dates specified. The showers that are to be seen after midnight are, unless of special note, omitted. 132 Meteors, or Shooting Stars There are, besides the meteors that have been classified, certain shooting-stars that apparently have no determined radiant point. These are called sporadic meteors. In these lists of meteors, the radiant point is only approximately given; for scientific purposes a far more exact position is required in terms of right ascension and declination. There are several good lists of meteoric showers to be obtained, which afford this information for those who care to pursue the matter more in detail. See the Rev. T. W. Webb's book, entitled Celestial Objects for Common Telescopes. For purposes of identification, the radiant points here given will be found for the most part sufficient. |